Zinc Complexes Supported by Maltolato Ligands: Synthesis, Structure, Solution Behavior, and Application in Ring-Opening Polymerization of Lactides

Abstract

A series of novel zinc alkoxides supported by chelating maltolato (MalO; MalOH = maltol) ligands were successfully synthesized and characterized. Reaction of MalOH with ZnEt 2 (3:4) gives a trinuclear cluster [Zn 3 (Et) 2 (MalO) 4 ] ( 1 ), which spontaneously disproportionates in solution to mononuclear species [Zn(MalO) 2 ] ( 1a ) and [Zn(Et)(MalO)] ( 1b ); ( 1a ) and ( 1b ) form [Zn(MalO) 2 (py)] ( 2 ), [Zn(Et)(MalO)(py)] ( 3 ), [Zn(MalO) 2 ] 2 ( (1a) 2 ), [Zn(MalO)(OBn)] 2 ( (1c) 2 ), and [Zn 4 (Et) 2 (OEt) 2 (MalO) 4 ] ( 4 ) on addition of pyridine, benzyl alcohol (BnOH), or dry O 2, respectively. Compounds 1, 2, (1a) 2, and 4 were characterized by elemental analysis, NMR, ESI-MS, and single-crystal X-ray structural analysis. Variable-temperature NMR experiments showed that (1a) 2 and (1c) 2 are in equilibrium with the monomeric form in solution. The addition of l -lactide ( l -LA) to a combination of 1 and 2 equiv of BnOH in dichloromethane at room temperature in different molar ratios leads to rapid and efficient generation of poly( l -LA) with end-capped BnO groups. According to kinetic studies, propagation by [Zn(OBn)(MalO)] ( 1c ) is first-order with respect to both the monomer and 1c concentrations; 1a in ring-opening polymerization of l -LA shows no activity. These results suggest a single-site active species in the ring-opening polymerization of l -LA.